Object.keys() is a method that returns an array of keys from the object it is called on. In your code, you are accessing the first element of this array, which corresponds to the first key in data.data.modelState.
The purpose of this code snippet is to retrieve the value associated with the first key in data.data.modelState.
For Example
Let's consider the scenario where:
data.data.modelState={tmpkey:"Some Value"}
var msg = data.data.modelState[Object.keys(data.data.modelState)[0]];
console.log(Object.keys(data.data.modelState)[0]); //will Print ["tmpkey"]
console.log(msg); //It will print "Some Value";
You can access any key of an object using square brackets []; in this case, we are focusing on the first key.
var msg = data.data.modelState[Object.keys(data.data.modelState)[0]];
In essence, this line of code can be simplified as follows:
var msg = data.data.modelState[["tmpkey"][0]];
Which further simplifies to:
var msg = data.data.modelState["tmpkey"]; //This directly retrieves the value associated with the "tmpkey" property.